Megasoma pachecoi Cartwright, 1963

Ratcliffe, Brett C. & Morón, Miguel-Angel, 2005, Larval Descriptions Of Eight Species Of Megasoma Kirby (Coleoptera: Scarabaeidae: Dynastinae) With A Key For Identification And Notes On Biology, The Coleopterists Bulletin 59 (1), pp. 91-126 : 93-96

Megasoma pachecoi Cartwright , 3rd instar ( Figs. 1–15 )

Description based on remnants of two third instar larvae reared from eggs laid in captivity by females collected in Mexico: State of Sonora, near Ciudad Obregón, IX / X- 1964 on branches of Cercidium torreyanum , 120 m, W.W. Gibson collector (UNSM, IEXA).

Description. Head. Maximum width of head capsule 11.3 mm. Surface of cranium densely punctate, dark brown. Frons ( Fig. 1 ) on each side bearing 1 posterior frontal setae, 7 anterior frontal setae, and 2 anterior angle frontal setae; remaining cranial surface with 4 dorsoepicranial setae, 1 epicranial seta, and 5–6 paraocellar setae at each side. Clypeus with 2 lateral setae on each side. Labrum slightly asymmetrical, ovate, with 3–4 posterior setae, 2 central setae, and 2 lateral setae. Ocelli not evident. Epipharynx ( Fig. 2 ) with fused zygum and epizygum forming oblique, raised, sclerotized tubercle on haptomerum; heli and plegmatia absent; right chaetoparia with 42–47 spinelike setae and 60–65 sensillae toward the gymnoparia; left chaetoparia with 56–60 spine-like setae and 52–58 sensillae; acroparia with 6–7 straight, long, thick setae toward right side and 10–12 medium size, slender setae toward left side; right acanthoparia with 9–10 short, curved, spine-like setae; left acanthoparia with 13–14 short, curved, spine-like setae; pedium wide, epitorma vague; dexiotorma long, narrow, with acute inner side; laeotorma shortened, with wide, rounded pternotorma; dexiophoba sparse, formed by 4–6 setae; laeophoba well-developed between haptolachus and inner side of laeotorma, formed by 12–17 setae; sclerotized plate of right nesium large, elongate, transversal; sense cone on left nesium represented by large, longitudinal, wellsclerotized plate; crepis not defined. Scissorial area of right mandible ( Fig. 3 ) slightly abraded, with 2 large, wide teeth, well-separated by notches, and 1 basal, small tooth; molar area with 3 lobes, brustia sparse. Scissorial area of left mandible ( Fig. 4 ) slightly abraded, with 2 well-marked, large teeth and 1 rounded pre-incisor tooth; molar area with 2 wide lobes, sparse brustia and long, truncate acia. Stridulatory area of each mandible well-marked by 30–32 fine striae. Mala ( Fig. 5 ) with apical uncus of galea well-developed and 3 conical unci fused at their bases at apex of lacinia ( Fig. 6 ); maxillary stridulatory area ( Fig. 7 ) with 8 small, subtrapezoidal, rounded or truncated teeth and wide, truncate anterior process. Labium with numerous long, strong setae located principally at sides; hypopharynx with right anterior lateral process of hypopharyngeal sclerome strongly developed, raised and curved, heavily sclerotized; left hypopharyngeal lateral lobe with patch of 10– 12 posterior lateral setae ( Fig. 5 ). Last antennal segment on dorsal surface ( Fig. 8 ) with 10–14 sensory spots; ventral surface with 10–12 spots ( Fig. 9 ). Thorax. Pronotum with wide, irregular, reddish brown lateral scleromes and 1 transverse row of 32 long, slender setae. Meso- and metanotum each with scattered, long, slender setae at middle and 30 setae on each side. Thoracic spiracles 1.3 mm long and 2.0 mm wide; lobes of respiratory plate contiguous, directed posteriorly. Spiracular bulla prominent, transverse, with acute, erect process. Tarsal claws or tarsunguli ( Figs. 10–11 ) similar on all legs, with acute apex, each bearing 2 internal, long, thick, basal setae and 2 external, long, thick, preapical setae. Abdomen. Dorsa of segments I–VI each with 2–4 transverse rows of 20–36 long, slender setae and 3–5 transverse rows of 40–65 short, spine-like setae. Dorsa of segments VII–VIII with 3–4 rows of 20–28 long, slender setae and 4–6 rows of 40–46 short, spine- 94 THE COLEOPTERISTS BULLETIN 59(1), 2005

like setae. Dorsum of segment IX with scattered, short, spine-like setae in anterior half and 50–60 long, slender, setae on sides where cuticle is slightly rugose.Dorsum of segment X with 60–65 short, spine-like setae and 40–45 long, slender setae on each side.Venter of abdominal segments I–IX each with 1 median, transverse row of long setae. Raster without pallidia and septula, with 8 long setae in campus, teges formed by 50 setae; cuticle longitudinally rugose. Lower anal lip with 110–120 short, spine-like setae and 5–7 medium size, erect setae. Upper anal lip with 120–130 short, spine-like setae. Spiracles of segments I–VIII progressively larger (1.5–1.8 mm wide; 1.1–1.4 mm long), lobes of respiratory plates contiguous ( Fig. 12 ), directed forward. Spiracular bulla prominent, transverse, with acute, erect process ( Fig. 13 ); respiratory plate with 47–54 irregularly-shaped holes ( Fig. 14 ) across any diameter. Approximate dorsal body length not measurable.

First-instar Larva. Based on one first-instar larva reared from egg laid by female collected in México: Sonora, Tezopaco , 10-IX-2002, G. Nogueira, fixed at 18 days age ( IEXA) .

Description. Similar to third-instar larva except as follows: maximum width of head capsule 2.8 mm. Epicranium and frons with many long, slender setae. Last antennal segment with 5–6 dorsal sensory spots. Sclerotized plates on sides of pronotum absent. Eclosion spine on each side of metanotum small, narrow, conical, with acute apex. Thoracic spiracles 0.12 mm wide, 0.11 mm long. Abdominal spiracles kidney-shaped, similar in size: 0.12 mm wide, 0.10 mm long, without definite bulla. Approximate dorsal body length 12 mm.

Remarks. Larvae of M. pachecoi share many characters with the larvae of M. elephas such as: 4 dorsoepicranial setae, 2 lateral clypeal setae, rounded pternotorma, 8 maxillary stridulatory teeth, acanthoparia with 8–10 setae, spine-like setae on dorsum of second abdominal segment, abdominal spiracles progressively longer from segments I–VIII, and lobes of respiratory plates of spiracles contiguous. However, in M. pachecoi the structure of the prominent, raised, acute bulla, 10–14 dorsal sensory spots on the last antennal segment, reduced number of setae on the right chaetoparia, labrum, epicranium and frons, as well as the head and body size, separate the larvae of these species.

Distribution and Habits. Adults of M. pachecoi have been collected in thorn bush and deciduous tropical forests located between 100 to 500 meters elevation at the southern borders of the Sonoran desert, from Esperanza east to Tezopaco, Hornos, Ciudad Obregón, Valle del Yaqui and Navojoa (all state of Sonora), then south to El Carrizo and 35 mi N Los Mochis (state of Sinaloa), México ( Cartwright 1963; Gibson 1964). Adults are crepuscular and are attracted to electric lights, but during the day, 96 THE COLEOPTERISTS BULLETIN 59(1), 2005

usually under very hot conditions [more than 42 8 C]), they are active on the branches of

‘‘palo brea,’’ ( Cercidium torreyanum (Wats.) Sarg. , Caesalpinaceae), or ‘‘huacaporo,’’ ( Parkinsonia aculeata L., Caesalpinaceae). Adults are active between August and October and feed on the cambium or waxy secretions of the branches. Males frequently engage in combat ( Fig. 15 ), pushing and shoving with their horns on the same branch for some minutes until one abandons the site (G. Nogueira, pers. comm., to MAM). Gibson (1964) found larvae inside unidentified, rotten stumps and in the soil of an old corral. He also reared larvae and adults from eggs laid by females in captivity that, together with males, were collected between mid-September to mid-October and maintained in cages with overripe bananas. Copulation lasted at least 15 minutes, and females began to oviposit immediately thereafter. Females from the field contained 1– 23 eggs, and the average was 13. The pearly, oval eggs, averaging 4.4 3 2.9 mm in size, were laid singly in galleries excavated in compost supplied to the females. Eggs laid in late September hatched in 29 days (average) whereas those laid in mid-October hatched in 37 days (average). Larvae were 8 mm in length at eclosion and grew about 1.5 mm per week on a diet of compost and rotten wood. They seemed tolerant of a lack of food and water. A few 80 mm long larvae pupated at ten months of age and emerged after three weeks pupation as undersized adults. The remaining larvae needed at least two years to complete their life cycle ( Gibson 1964).

Figures

Figs. 1–14. Megasoma pachecoi, third instar. 1) Head, frontal view; 2) epipharynx; 3–4) mandibles, ventral view, 3) right and 4) left; 5) right maxilla and hypopharynx, dorsal view; 6) detail of unci of lacinia; 7) detail of maxillary stridulatory area; 8–9) distal segment of antenna, 8) dorsal view and 9) ventral view; 10–11) left tarsal claws, external views, 10) middle and 11) posterior; 12) second left abdominal spiracle; 13) detail of bulla of spiracle, ventral view; 14) microstructure of respiratory plate (5003). Figs. 1–5, bar ¼ 1 mm. Figs. 6–13, bar ¼ 0.5 mm.

Fig. 15. Megasoma pachecoi, males fighting on branch of Cercidium torreyanum. Tezopaco, Sonora, Mexico. August 1998. Photo by G. Nogueira.